Digital Contrast Control

[TheAnimus]

In one of my current projects i want to control the contrast of a graphical LCD from a PIC16F870, pins are getting short, as such when doing the first design i just thought digital pot. only 2 wires interface, however i have been unable to find any serial interfacing ones that can act as a potential divider over the required voltage range (0v to -13.8v).

Thing is the Digital Potentiometers i've seen, well all 4 that looked remotely suitable from RSwww.com apears too only be able to have voltage below Vdd and above Vss (understandably).

So trying to keep component count low, any ideas on the best option?

 

[The Real MicroMan]

Try sending a PWM signal out of you PIC, then to an op-amp, then to a low pass filter. You should be able to get 0-13v that way.

 

[TheAnimus]

Hey thanks for ur fast reply, i forgot to say, i have the -13.8v generated by the LCD, i am very keen to keep the component count down, and i was hopeing i could use the PWM for adjusting the CCFL inverter intensity.

edit: before my edit you would not of belived english is my first langauge!

 

[TheAnimus]

Just an uptdate for anyone in future, i have decided that the simplest way is a serial DAC and a bi-polar transistor!. Thou i am not sure about my biasing trail and error will be rife!

 

[Chippie]

Just an uptdate for anyone in future, i have decided that the simplest way is a serial DAC and a bi-polar transistor!. Thou i am not sure about my biasing trail and error will be rife!

But isnt that what experimenting is all about ? :lol:

 

[TheAnimus]

lol indeed its just while i am not at uni (xmas holidays) getting components is a bit tricky. At my perants in cornwall (very remote, most south west part of England, water on 3 sides) getting to somewhere as mundian as a maplin's is a 2 hour quest.

I can't belive no one here hasn't ever added a digital contrast control to an LCD hooked upto a PIC or somthin'! i will have to make sure to post my final design!

 

[Nigel Goodwin]

I can't belive no one here hasn't ever added a digital contrast control to an LCD hooked upto a PIC or somthin'! i will have to make sure to post my final design!

I've never done it, but I have seen it done - simply by using a PWM output from a PIC pin to feed the contrast pin on the LCD (which requires a 0-5V range) - from what I can remember (and I wasn't particularly paying attention!) there wasn't any low-pass filtering used, just the direct PWM.

 

[panic mode]

I use LCD which also has 0-5V contrast control. Even so
I never bothered to control contrast by PIC, I simply put
a potentiometer. PWM would be a way to go if you really want it
controlled by PIC. I believe the simplest way change polarity
and signal level would be to use an optocoupler.
And if you want to experiment, do you really have to go to
'local' store for parts? It sound like expencive way to play
around. I'm sure you have some electronics laying around so
you could salvage few components. At least that's what I do.
Used computer hardware is cheap or free just to name one
good source of parts. I buy parts as well but not nearly enough
to do decent project.

 

[TheAnimus]

yeh i am using a graphical LCD in this project, last time i was using a HD44780 i just used the built in PWM but regratbly i am already using that on this project, and can't afford the CPU overhead of a TMR based PWM. I think the idea of an opto-icolator is a good one, because i was planning on using a DAC that could ouput 0v -2v (some strange level) but that might be a lot cheaper!

As for scavenging, my perants made me take all that stuff with me to uni, i can't be bothered to pack it all up for the 4 week holiday!

 

[pike]

head over to www.analog.com.

You can obtain samples of digital pots there for free.The only problem i got when the package arrived was that they were SMD types.

 

[TheAnimus]

thx pike i will give that a shot